Display devices which can display stereoscopic images are now making significant progress. Those stereoscopic image display devices can be roughly categorized into those to be viewed with glasses and those to be viewed with naked eyes. Especially, the display apparatuses by which stereoscopic images can be viewed with naked eyes do not bother users to wear glasses, and are expected to come into wide use. As the stereoscopic display device by which naked eyes can perceive stereoscopic images, display devices which employ cylindrical lenses or a parallax barrier arranged in front of a display unit are generally used. In those display devices, pixels for the right eye and pixels for the left eyes are prepared in the display unit and those pixels are arranged so as to transmit information of each pixel to eyes of an observer through the cylindrical lenses or the parallax barrier. Therefore, the cylindrical lenses are arranged such that the direction of cylinders agrees with the vertical direction of the screen of the display unit, and the parallax barrier is arranged such that the direction of its light-shielding slits agrees with the vertical direction of the screen.
On the other hand, small-sized liquid-crystal display units are now widely used in the field of portable handheld devices such as portable game machines and mobile phones. Since those portable handheld devices are developed on the assumption that they are driven by batteries, display units with smaller power consumption are required in the field of portable handheld devices. In the field of portable handheld devices, the way to use a device such that the device is turned in a user's hand to change its screen from the portrait orientation to the landscape orientation, and vice versa, according to an application program executed thereon, has been widespread. Folding mobile phones which can be used by turning just a display unit by 90 degrees so as to change its screen from the portrait orientation to the landscape orientation are now in the market. In the field of smart phones, devices which can switch the screen between the portrait orientation and the landscape orientation freely by turning the device in a user's hand from the portrait position to the landscape position, are becoming popular.
In view of the above situation, development of display devices which can switch the screen between the portrait orientation and the landscape orientation and can display stereoscopic images in both of the portrait-orientation use and landscape-orientation use, have got to be considered. As a typical technology to realize such display devices, the technology disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2010-170068 has been proposed. This technology will be described with reference to FIG. 23. There is provided a display unit composed of lens array element 24 and display panel 25. Lens array element 24 is a switching array element using liquid crystal, and includes first electrode group 22 extending in the X-direction on a lower substrate and second electrode group 23 extending in the Y-direction on an upper substrate. FIG. 24 illustrates a detailed perspective view of the lens array element 24. In FIG. 24, there are provided electrodes 27 and 26 with two types of width (Lx, Sx) extending in the X-direction on one of substrates. On the other of the substrates shown in FIG. 24, there are provided electrodes 28 and 29 with two types of width (Ly, Sy) extending in the Y-direction. Molecules of liquid crystal are oriented in one direction initially.
The above-described electrodes can be driven in three states illustrated in FIGS. 25A to 25C. FIG. 25A illustrates the first state that an electric potential gradient is generated in the X-direction and a uniform electric potential distribution is generated in the Y-direction. Therefore, the alignment of liquid crystal molecules has a cyclic distribution in the X-direction. One cycle of the distribution is defined by a distance from one electrode with width Ly to the next electrode with width Ly, as one unit. Such the structure provides a liquid crystal lens which works in the optically same manner as cylindrical lenses extending in the Y-direction and each having the same width as the above distance. When a display panel having pixels for the left eye and pixels for the right eyes arrayed in the X-direction is layered on the lens array element in the above state, a stereoscopic image display device wherein the vertical direction of the screen agrees with the Y-direction can be obtained.
Next, FIG. 25B illustrates the second state that no voltage difference is made between the upper and lower substrates and molecules of liquid crystal are oriented parallel with the substrates. Since the liquid crystal does not have a refractive-index distribution in this state, the liquid crystal works as a transparent body. When a display panel is layered on the lens array element in this state, the display panel works as a normal display panel for 2D display.
Further, FIG. 25C illustrates the third state that an electric potential gradient is generated in the Y-direction. Such the structure provides a liquid crystal lens which works in the optically same manner as cylindrical lenses extending in the X-direction and each having a width from one electrode with width Lx to the next electrode with width Lx as one unit. Thereby, when a display panel having pixels for the left eye and pixels for the right eyes arrayed in the Y-direction is layered on the lens array element in the above state, a stereoscopic image display device wherein the vertical direction of the screen agrees with the X-direction can be obtained.
As described above, use of a lens array element having the structure illustrated in FIG. 24, allows a display device to carry out the three types of display including the stereoscopic image display whose vertical direction agrees with the Y-direction, the normal 2D display, and the stereoscopic image display whose vertical direction agrees with the X-direction.
However, the lens array element illustrated in FIG. 24 hardly realizes the same lens operations in both of the X-direction and the Y-direction. This problem will be described with reference to FIGS. 26A and 26B. FIG. 26A is a sectional view illustrating a molecular alignment state when no voltage is applied to the lens array element and the lens array element shown in FIG. 24 is viewed from the X-direction. In FIG. 26A, molecules of liquid crystal have been oriented in the Y-direction in the initial state, and they have an alignment almost parallel with the substrates at a very small pre-tilt angle with the substrates.
FIG. 26B is a sectional view illustrating the molecular alignment state when a voltage is applied between the upper and lower substrates of the above lens array element. In FIG. 26B, since a voltage is applied to the liquid crystal layer between the substrates, the molecules are rotated toward the vertical orientation from the orientation of the substrate surfaces. Since this molecular state is defined based on the direction of the initial pre-tilt angle, the molecular alignment becomes asymmetric about the center of the electrode with width Sx as shown in FIG. 26B. As can be seen from FIG. 26B, the state of liquid crystal molecules on applying a voltage thereto becomes asymmetric, while the electrode structure is symmetric about the center of the electrode with width Sx. Such the lens array element merely forms asymmetric cylindrical-lens structures extending in the X-direction above the electrodes with width Sx.
On the other hand, FIG. 27A illustrates a molecular alignment state when no voltage is applied to the lens array element and the lens array element shown in FIG. 24 is viewed from the Y-direction. Since molecules of liquid crystal have been oriented in the Y-direction in the initial state, the molecules along the electrodes with width Sy have an alignment which has symmetry from the initial state. Therefore, also in the state that a voltage is applied between the upper and lower substrates as illustrated in FIG. 27B, the symmetric alignment of liquid crystal molecules about the center of the electrode with Sy width can be obtained. Such the lens array element forms symmetric cylindrical-lens structures above the electrodes with width Sy.
As described above, it can be found that when the liquid crystal molecules have been oriented in the Y-direction in the initial state, only asymmetric cylindrical-lens structures, which extend in the X-direction, are provided in the third state among the three states shown in FIGS. 25A to 25C. Further, it can be found that when the liquid crystal molecules have been oriented in the X-direction in the initial state, only asymmetric cylindrical-lens structures, which extend in the Y-direction as a perpendicular direction to the initial orientation direction of the liquid crystal molecules (the X-direction), are provided in the first state among the three states shown in FIGS. 25A to 25C.
When a stereoscopic display device carries out 3D display by using the above asymmetric cylindrical-lens structures, it is difficult to perform a separation of light to the left eye and the right eye of an observer properly. It results in an increase of the amount of 3D crosstalk and in a brightness difference between light perceived by the right eye and light perceived by the left eye. Therefore, it is difficult for such the stereoscopic display device to realize stereoscopic image display of high quality. Further, when the degree of the asymmetry is significant, it becomes impossible for such the stereoscopic display device even to present stereoscopic images for observers.